單層/多層二硫化鉬橫向異質接面的光電特性

Abstract

二硫化鉬自旋/能帶谷的自由度，及落在可見光範圍的能隙，使得此材料在光電應用上具有前瞻性。最近已有許多團隊使用過渡金屬硫族化物( TMD )材料形成P-N 接面。在本論文中，我們將展示建立在二硫化鉬上的新結構–橫向異質接面。此橫向異質接面的結構是基於單層、多層二硫化鉬能帶結構本質上的差異。我們使用剝離法得到單層連著多層的二硫化鉬薄片，此薄片即可形成橫向的異質接面元件。內建電場的存在被驗證於克爾文力顯微鏡(KPFM)、截止狀態的光電流分佈、源極-汲極電壓相關的電流分布，及不對稱的電流密度-電壓曲線。光伏效應可見於無外加電壓下的光電流分佈及照光下的電流密度-電壓曲線。迅速的光開關電流變化，及穩定的表現也在實驗中被觀察到。從閘極電壓相關的電流分佈及電流密度-電壓曲線中，我們發現可用全域閘極電壓來調變能階校準。在單層/多層交界的電流大小隨著閘極電壓下降而增強，以及因著閘極電壓上升而變線性的電流密度-電壓曲線，指出單層二硫化鉬的費米能階對閘極電壓的變化較大。此實現於二維新穎TMD 材料上的橫向異質接面結構，及使用全域閘極電壓調變能階校準的可行性開啟了通往物理研究及科技應用的門。

The spin/valley degree of freedom and the bandgap in visible light range make MoS2 a promising material for optoelectronic applications. Several groups have reported construction of p-n junction based on Transition-Metal Dichalcogenide (TMD) recently. In this thesis, a new architecture – lateal heterojunction – based on MoS2 is demonstrated. The optoelectronic properties of the lateal heterostructure is determined by the difference of intrinsic band structures of monolalyer and multilayer MoS2. The lateralheterojunction devices are realized by choosing monolayer/multilayer MoS2 after mechanical exfoliation. The existence of built-in field is examined by Kelvin Probe Force Microscope (KPFM), photocurrent mapping at off state, bias-voltage dependent photocurrent mapping, and the asymmetric J-V curve. The photovoltaic effect is observed by performing photocurrent mapping and J-V curve under illumination. Prompt optical switching with robust performance are observed. From the back-gate voltage dependent photocurrent mapping and J-V curves, we achieve the modulation of band alignment by global gating. We observed the enhanced photocurrent at the interface as back-gate voltage lowered and the J-V curve grew linear as back-gate voltage increased, both suggesting that the the Fermi level of monolayer MoS2 is more sensitive to the back-gate voltage. The realization of the lateral heterojunction based on the novel two-dimensional TMD materials and the band alignment modulation by global gating open the avenue to both physical research and technological applications.